A few months ago, someone on this forum mentioned the bearings holding the differential wear out at around 200k miles. A bad differential bearing means the differential ends, where the axles are inserted, will move around instead of staying dead center relative to the axle seals which results in a leak. (I know because my transmission needs the same repair; the driver's side seal leaks; it's new; I replaced both axle seals when I did the clutch about 1k miles ago.)

He also said that it is possible to replace the differential bearings without removing the transmission.

That could be your problem.